function showHillAndTitleAnimation() {
	var class_about = document.querySelector(".about");
	if(isElementFullVisual(class_about, 0.6)) {
		var class_about__title = document.querySelector(".about__title");
		var class_about__intro = document.querySelector(".about__intro");
		class_about.style.backgroundPositionY = "100%, 100%, 100%";
		class_about.style.opacity = "1";
		class_about__title.style.top = "0";
		class_about__intro.style.top = "0";
		window.removeEventListener("scroll", showHillAndTitleAnimation, false);
	}
}

function showMainNav() {
	var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
	var class_mainNav = document.querySelector(".main-nav");
	if(scrollTop >= 48) {
		class_mainNav.style.opacity = 1;
	} else {
		class_mainNav.style.opacity = 0;
	}
}

window.addEventListener("scroll", showHillAndTitleAnimation, false);

window.addEventListener("scroll", showMainNav, false);

showHillAndTitleAnimation();